The Orthogonal Frequency Division Multiplexing (OFDM) technology can overcome frequency selective fading in a broadband mobile channel with low complexity, and thus is widely used in various broadband mobile communication systems. In order to correlatively detect each OFDM data subcarrier symbol, a pilot plays an important role in the system. Through the pilot symbols, the receiver estimates a wireless channel H, and then assists the equalizer or demodulator to equalize the channel or related detected data symbols. In addition to the correlation detection or demodulation function, the pilot is used in the system for measuring the quality or state of the wireless channel, and aiding the scheduler to implement functions such as frequency selective scheduling and link adaptation.
The Multiple Input Multiple Output (MIMO) multiple antenna technology can improve the reliability and capacity of a wireless communication system by utilizing space scattering characteristics of the wireless propagation channel, and thus is also applied widely in various types of wireless systems. The MIMO-OFDM technology has become the default configuration in a broadband mobile communication system. In recent years, the function of the pilot is more refined in the MIMO-OFDM system. For example, the LTE-Advanced system specifically sets a pilot and a measurement pilot, which are respectively used for the system correlation detection and channel measurement functions. The reason for this design is that a multi-antenna pre-coding technology is used in MIMO systems, and particularly when the pre-coding matrix is unknown by the receiver, the pilot has to be pre-coded along with data symbols, but the pre-coding per se can somewhat change frequency domain characteristics of the mobile channel. Therefore, the pilot and the measurement pilot have to be separated.
In various existing mobile communication systems or wireless LAN systems, the pilot is usually fixedly configured in the system according to a certain pattern. Take an LTE-Advanced system as an example, in each time-frequency Resource Block (RB), the pilot configuration is shown in FIG. 1, wherein a physical downlink control channel (PDCCH) in the LTE system is used to allocate various resources for uplink and downlink transmission in the entire system, and plays an crucial schedule role in the system. The physical downlink shared channel (PDSCH) is used for transmission of signaling at the service or control plane, where CRS is a common pilot and DMRS is a dedicated pilot. Depending on spatial data streams transmitted in parallel, the number of ports for the pilot may vary, but the time domain pilot density and the frequency domain pilot density are constant which has been determined in the system specifications.
In 802.11 wireless LAN systems, the pilot is also fixed in each physical frame header, i.e. a long training sequence. Regardless of the length of the transmission cycle, the propagation environment and the used transmission format, the configuration of the pilot does not change.
As well known, the mobile channel is complex and changeable, and in different propagation environments, the mobile channel's frequency selective fading, time selective fading and space selective fading will be significantly different. Using pilot in fixed pattern is not conducive to its adaption to a complex, and changing mobile communication environment, and further causes certain loss in the system capacity. Take LTE-Advanced as an example: when the terminal operates in indoor environment, due to its lower moving speed, the mobile channel has a longer correlation time (>10 ms). However, whatever the correlation time is, there will be a constant repetition for the pilot in each sub-frame (1 ms) in LTE-A system. Take 802.11 system as an example again: when the system works in outdoor hotspot, due to the rapid change of surrounding environment, for example: the movement of the car, even if the terminal is stationary, the channel between the AP and the terminal will still have a Doppler spread, and thereby forming time selective fading. But no matter how the environment changes, the 802.11 system's pilot function will be borne by the long training sequence which has a fixed location in the physical frame. Thus, the fixed pilot can't adapt to environmental change.